We are fixing an issue with bloom in the next update. This issue works on texture fillrate.

285 gtx texture fillrate : 51.8

580 gtx texture fillrate : 49.4

So that would actually be worse. However memory bandwidth and vertex processing is much faster.

285 gtx memory bandwidth : 159.0

580 gtx memory bandwidth : 192.4

Cude cores:

285 gtx : 240

580 gtx : 512

This means bigger textures are less of a problem and that really large polygon models should be better.

If we fix the bloom issue that hogs texture fillrate a 580 gtx is probably not a bad choice. However a fast cpu lets you run models with more materials.

So it is hard to say if the upgrade is huge enough.

For someone without a card like 285 gtx the 580 gtx is a clear winner. If you do have one you should probably first test your scene on a 580 before buying to see if it gives the speed where you need it.